Spatial and seasonal distribution of selected nitrogen cycle genes in deep waters of the Baltic Proper

Michał Grabski ^1* Ewa Kotlarska ² Aneta Łuczkiewicz ³ Konrad Hryniewicz ⁴ Grzegorz Wegrzyn ¹ Beata Szymczycha ^2*

• ¹ University of Gdansk, Gdansk, Pomeranian Voivodeship, Poland
• ² Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
• ³ Gdansk University of Technology, Gdansk, Pomeranian, Poland
• ⁴ Gdynia Maritime University, Gdynia, Poland

Marine nitrogen cycle ultimately depends on the biological responses of oceanic microbial communities. It indirectly regulates primary production and influences the strength of the biological pump, which contributes to the oceanic uptake of atmospheric carbon dioxide (CO₂). As the microbial community structure and functional capacities remain underestimated in terms of temporal and geographical coverage in the Baltic Sea our understanding of the nitrogen cycle with respect to ecosystem functioning and climate change is limited. Therefore, in this study, we investigated the seasonal and spatial structure of microbial community abundance involved in the nitrogen loss (denitrification, anammox), reduction processes (dissimilatory nitrate reduction (DNR), dissimilatory nitrite reduction to ammonium (DNRA)) and oxidation process (nitrification) in the Baltic Proper (Bornholm Deep, Gdańsk Deep, and Gotland Deep). Specifically, we focused on waters below the halocline at depths ranging from 75 to 135 meters, characterized by changeable oxygen conditions. The potential of selected nitrogen processes was resolved by mapping raw reads against nitrogen cycle genes identified in de novo assembled metagenomes. Taxonomic analysis of bacterial and archaeal communities, based on paired-end raw reads, revealed that nitrification, DNR, and denitrification potential were primarily associated with the Nitrosopumilaceae and Thioglobaceae families within these phyla. Ammonia oxidation products likely fueled the production of nitrous oxide (N₂O), with nitric oxide reductase (NOR)—an enzyme encoded by the Thioglobaceae genome—being responsible for further reduction. Anammox related genes were not present within sites, thus denitrification pathway enzymes namely NOR and N2O reductase (NOS) were responsible for nitrogen loss. At all sites genes encoding nitrogen reduction enzymes were most abundant while the presence of NOS encoding genes was found in Bacteroidetes and Proteobacteria phyla within all sites. Our findings revealed no significant spatial variation, suggesting that the studied ecosystem exhibits a consistent nitrogen processing capacity across different locations. However, seasonality emerged as a key factor, as changes in nutrient and oxygen conditions throughout the year significantly influence microbial activity and the associated nitrogen-cycling processes.